How to append to bytes in python 3

Question

I have some bytes.

b'x01x02x03'

And an int in range 0..255.

Now I want to append the int to the bytes like this:

b'x01x02x03x05'

How to do it? There is no append method in bytes. I don’t even know how to make the integer become a single byte.

>>> bytes(5)
b'x00x00x00x00x00'

Asked By: felixbade

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Source

Answer 1

bytes is immutable. Use bytearray.

xs = bytearray(b'x01x02x03')
xs.append(5)

Answered By: simonzack

Answer 2

First of all passing an integer(say n) to bytes() simply returns an bytes string of n length with null bytes. So, that’s not what you want here:

Either you can do:

>>> bytes([5]) #This will work only for range 0-256.
b'x05'

Or:

>>> bytes(chr(5), 'ascii')
b'x05'

As @simonzack already mentioned, bytes are immutable, so to update (or better say re-assign) its value, you need to use the += operator.

>>> s = b'x01x02x03'
>>> s += bytes([5])     #or s = s + bytes([5])
>>> s
b'x01x02x03x05'

>>> s = b'x01x02x03'
>>> s += bytes(chr(5), 'ascii')   ##or s = s + bytes(chr(5), 'ascii')
>>> s
b'x01x02x03x05'

Help on bytes():

>>> print(bytes.__doc__)
bytes(iterable_of_ints) -> bytes
bytes(string, encoding[, errors]) -> bytes
bytes(bytes_or_buffer) -> immutable copy of bytes_or_buffer
bytes(int) -> bytes object of size given by the parameter initialized with null bytes
bytes() -> empty bytes object

Construct an immutable array of bytes from:
  - an iterable yielding integers in range(256)
  - a text string encoded using the specified encoding
  - any object implementing the buffer API.
  - an integer

Or go for the mutable bytearray if you need a mutable object and you’re only concerned with the integers in range 0-256.

Answered By: Ashwini Chaudhary

How to append to bytes in python 3

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